1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1145 
we see the first parthenogenetie females containing 30-40 eggs (fig. 
1) ; and on 1st July besides those a few young ones derived from 
ephippia or from parthenogenetie females. On 15th July we find 
many young small-sized animals, males and females (figs. 2 and 3), 
but only a few of the latter have eggs (2-3) and no ephippia ; further, 
we find some females of larger size, all with eggs (often 12) (fig. 4). 
In all these samples the number of D . longispina has only been 
small; on 15th July we find for the first time the two sizes of 
females which now will be found in all the following samples. The 
small females are only c. T5 mm., the large c. 2*5 mm. in length. 
On 1st and 15th August the number of animals is suddenly 
enormous ; most of them are young animals, males and females 
(fig. 5), of which a few have ephippia (fig. 7). The large females 
from 1 5th July are now almost all barren ; but on 15th August I have 
found a few with ephippia ; a great many newly hatched young ones 
appear. On 1st September we find almost only animals of the 
smaller size, some of them with ephippia ; males are rare. Of the 
large females I have only found but few and barren individuals. On 
18th September a great many of the small-sized females (fig. 10) 
have ephippia (fig. 8), but these are only half-developed and with- 
out lodges ; the males are rather common, and so are also the young 
ones. The large females are again very common and contain 8-10 
eggs (fig. 9). On 1st October the smaller-sized ones commonly have 
fully- developed ephippia ; still I noticed a few without these. A few 
